The present invention relates to the manufacture of foam sheet stock used in a wide variety of applications, such as for example containers, meat trays, packaging materials and antifriction place mats for airlines food service.
During the manufacture of foam sheet stock from materials such as polystyrene, polyethylene and the like, it is well known to introduce the basic polymer or copolymers into one or more extrusion devices in order to heat the polymer and incorporate therein certain nucleating agents, as well as the blowing agent. The thoroughly heated and masticated plastic material is then extruded through an extrusion orifice into a thin sheet or preferably a tube. When the extrudate takes the shape of a tube, it is drawn over a mandrel, thus expanding the circumferential extent of the tube. In addition, the tube of foam material is pulled away from the mandrel at a speed greater than the extrusion speed, thus inducing a certain amount of orientation into the foam sheet material. The orientation in a cellular foam sheet is a desirable feature in that certain memory characteristics can be built into the foam sheet. For example, it is now common to sever rectangular shaped pieces of foam sheet material and form them into cylinders having an overlapped liquid impervious seam. The cylinder thus formed is placed on a mandrel and subjected to controlled heat, thus causing the foam material to shrink and assume the configuration of the mandrel. Both one and two-piece drinking cups have been manufactured in this manner. Then too, a protective cover for bottles has been used for several years in the carbonated beverage field.
In order to monitor newly created foam sheet material, it is highly desirable to be able to examine in minute detail the actual cell structure within the sheet. Microscopic examination at, for instance, 60X magnification reveals several important aspects of how well the foam sheet has been fabricated. For example, it is highly important that the individual cells be of closed configuration if the ultimate purpose of the sheet material is for the fabrication of containers such as coffee cups and the like. A close-up examination of the cells within the sheet material reveals how well the surfaces of the foam material have been cooled. If the cooling is too rapid, small cell sizes will be created, thus acting as a bar to adequate cooling of the cells situated in the center of the sheet. Cells that receive inadequate cooling will have a tendency to rupture, thus reducing the overall integrity and usefulness of the sheet material. A good microscopic examination will reveal whether there has been an overload of the blowing agent and in the instance of a laminate, the skin thickness and uniformity can be monitored. A microscopic examination also permits an insight into the physical dimensions of each cell within the foam structure and its relationship with adjacent cells. As a foam material is generated soon after extrusion, the cells are normally spherical in configuration. With the introduction of orientation into the foam material, the originally spherical cells assume an elongate shape which they retain until subsequently released by the application of heat. Thus it becomes evident for several reasons to rely upon good microscopic examination of the individual cell structure in foam sheet material to assure adequate quality control.